In distance measuring methods measuring the time of travel of a measuring light pulse, it is difficult to correlate exactly in time the instant at which the time measurement is started with the instant, at which the measuring light pulse is emitted and, after its reflection at the target object, to correlate exactly in time the instant at which the time measurement is stopped, with the instant at which the measuring light pulse is received. Such exact correlation in time, however, is necessary to obtain a time measurement value, which, with great accuracy, corresponds to the time of travel of the measuring light pulse and thus makes possible an exact distance determination.
Particularly when targets within a great distance measurement range of, for example, 0 to 100 kilometers and more are to be measured at an accuracy of up to .+-.1 millimeter, extra-ordinarily great difficulties result, since especially for short distance targets, the delay and signal processing times which arise at the transmitter side between the trigger signal triggering the transmitter to generate a measuring light pulse and the actual emission of the light pulse as well as on the receiver side between the arrival of the reflected measuring light pulse at the receiver, and the generation of the associated stop signal lie in the same order of magnitude as or even exceed the actual "genuine" time of travel of the measuring light pulse.
If no appropriate measures are taken these delay and signal processing times fluctuate due to temperature changes and drift phenomena dependent on aging. These changes of delay and signal processing times can have such a strong effect on the measurement result that it becomes impossible to attain the aforementioned measurement accuracy.
The problem of establishing on the receiver side an exact correlation in time between the instant of reception of the reflected measuring light pulse and the instant of generation of the stop signal for the time measurement can be regarded as solved in principle by the circuit arrangement disclosed in DE-OS No. 26 34 627.
However, it is more difficult on the transmitter side to establish an exact correlation between the instant at which the transmitter responds to the trigger signal and a start signal since this response time or delay can be subject to strong changes. Therefore the trigger signal is not readily usable as a start signal for the time measurement.
To overcome this problem, it is for example known from the U.S. Pat. No. 3,652,161 to branch off a part of each measuring light pulse emitted by the transmitter, for example by means of a partially permeable mirror, and to use this branched off part as a reference light pulse which, via a reference light path, is forwarded to a photo-detector translating it into a start signal, correlated with the instant of transmission of the measuring light pulse. Two different receivers are used, the one receiving the measuring light pulses and the other one receiving the reference light pulses. If no further measures are taken, difficulties result from the different response behaviour and the different fluctuation phenomena of the signal processing times of the two receivers and of the two receiving channels connected thereto. If, on the other hand, measuring light pulse and reference light pulse are forwarded to one and the same receiver, the measurement of small distances becomes difficult, since the single receiving channel as well as the time measuring device can process without difficulties only signals which have a certain minimum spacing. Although it is possible with appropriate circuit effort to build time measuring device which can measure the time spacing of signals following one another practically as closely as desired, the amplifier and signal generating circuit of the receiving channel have a certain recovery time, i.e. a certain minimum time must elapse after the reception of a light pulse before a new pulse can be processed unobjectionably. This causes a lower limit for the smallest measurable distances, which limit can be made very small but can not be brought to zero.
An objective of the invention is to provide a method and an apparatus of the foregoing kind which is simple, reliable and operates with great accuracy over a wide measurement range, the lower limit of which is practically zero.